The present invention relates to a power steering system for assisting the steering by driving an oil pump through an electric motor.
With a typical power steering system, an oil pump is driven by an electric motor rotated in accordance with output of a torque sensor for sensing a steering torque, increasing the hydraulic pressure within one of the power-cylinder chambers separated by a piston, thus assisting the steering. The pressure difference occurs between the chambers by the steering made by the driver. With the pressure difference being greater, when the pump stops due to its anomaly and the like, the steering is locked.
In order to cope with such inconvenience, an electric hydraulic power steering system is proposed comprising a bypass circuit for providing fluid communication between the chambers and a fail-safe solenoid valve for switching between the communication state and the non-communication state of the bypass circuit. With this power steering system, when some anomaly occurs, the solenoid valve is switched to the communication state, canceling the pressure difference between the chambers, thus allowing non-assisted steering (refer hereafter to as “manual steering”) wherein a steering force is produced by the driver only.